I. Field
The following description relates generally to wireless communications systems, and more particularly to header compression systems and methods for wireless communication systems.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so forth. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems including E-UTRA, and orthogonal frequency division multiple access (OFDMA) systems.
An orthogonal frequency division multiplex (OFDM) communication system effectively partitions the overall system bandwidth into multiple (NF) subcarriers, which may also be referred to as frequency sub-channels, tones, or frequency bins. For an OFDM system, the data to be transmitted (i.e., the information bits) is first encoded with a particular coding scheme to generate coded bits, and the coded bits are further grouped into multi-bit symbols that are then mapped to modulation symbols. Each modulation symbol corresponds to a point in a signal constellation defined by a particular modulation scheme (e.g., M-PSK or M-QAM) used for data transmission. At each time interval that may be dependent on the bandwidth of each frequency subcarrier, a modulation symbol may be transmitted on each of the NF frequency subcarrier. Thus, OFDM may be used to combat inter-symbol interference (ISI) caused by frequency selective fading, which is characterized by different amounts of attenuation across the system bandwidth.
Generally, a wireless multiple-access communication system can concurrently support communication for multiple wireless terminals that communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Generally, each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. A MIMO system also supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows estimation of the forward link channel from the reverse link channel. This enables an access point to extract transmit beam-forming gain on the forward link when multiple antennas are available at the access point.
One portion of the data stream that is transmitted on various wireless networks relates to Internet Protocol (IP) headers. Due to the length of such headers, it is desirable to compress the headers to improve the efficiency of the network. Header compression (HC) is a method to compress IP packet headers. Several protocols exist to perform header compression, one example of which is the Robust Header Compression (RoHC) as specified in the IETF RFC 4095. For RoHC various profiles are specified including IP profiles which compress IP headers, including, but not limited to a User Datagram Protocol (UDP) profile for UDP/IP headers, a Real time Transport Protocol (RTP) profile for RTP/UDP/IP headers, and a Transmission Control Protocol (TCP) profile for TCP/IP headers. [Please add (ESP) profile for ESP/IP header compression to the list of RoHC profiles, where ever necessary].
Due to the extent of parsing, context lookup, storage, read/write operations required and so forth, the process of compressing and decompressing headers is computationally intensive. Also, due to the variety of operations involved, Header Compression (HC) is very often implemented in software rather than in hardware. While a terminal or other User Equipment (UE) may support 50 mega bits per second (Mbps) over the air on the downlink as well as an IP/Multimedia Subsystem (IMS) that includes RoHC IP, UDP, and RTP profiles, it is not reasonable to expect the UE to process 50 Mbps of header compressed data. Forcing the UE to support HC at bitrates advertised in the UE downlink/uplink (DL/UL) capability specification would cause such devices to be cost prohibitive. If more data is transmitted than the UE can decompress, the bandwidth spent transmitting the excessive data is diminished, since while such data are received at Layer 1 and acknowledged by Radio Link Control (RLC), HC cannot process the decompression rate and packets may be dropped within the terminal as buffers overflow. Worse yet, if many consecutive packets overflow before they are processed by the decompressor, the HC may become out of synchronization altogether.
When the HC is configured on the uplink (UL), similar reasons the UE may not be able to provide compressed at the rate advertised in the UE capability specification for. If the Evolved Node B (eNB) allocates UL resources up to the UE capability, these resources may not be properly utilized if the UE cannot compress fast enough. Digital padding may be used in order to fill the allocated transport block. One solution would be to disallow HC for flows with bit rates higher than a fixed threshold. However this would prevent future generations of UE from taking advantage of HC beyond the fixed threshold and also prevent potential, very efficient, HC implementations from utilizing the feature at high rates.